The present invention is related to rubber-modified monovinylidene aromatic polymer compositions.
It is well known in the art that interpolymers of monovinylidene aromatic monomers and ethylenically unsaturated nitrile monomers, such as styrene-acrylonitrile, can be made more impact resistant by the inclusion of elastomeric materials (rubbers) into the matrix or continuous phase of the composition. Typically, the rubber, such as polybutadiene, is in the form of discrete rubber particles having amounts of the matrix interpolymer graft-polymerized to the rubber particles.
It is also well known in the art that the physical properties of these types of compositions are greatly affected by the relative amounts of rubber particles having different sizes and particle structures or morphologies. Larger rubber particles having diameters greater than about 0.5 micron (.mu.m) generally improve impact resistance, but tend to reduce the gloss. Smaller rubber particles tend to increase gloss but offer less impact resistance. In addition, there is gloss sensitivity to consider, wherein gloss is reduced due to inadequate molding pressure in molding operations.
Additionally, it is known that grafted rubber particles containing occlusions of matrix polymer, provide more impact resistance than the same amount of rubber present as grafted, solid rubber particles. Such grafted, occlusion-containing rubber particles are usually produced in a mass polymerization process. Occlusion-containing particles produced in such mass processes are hereafter referred to as "mass particles".
Solid or non-occluded grafted rubber particles are typically produced by emulsion polymerization of a rubber in the form of an aqueous latex. The non-occluded type of rubber particles, produced via emulsion polymerization process, are hereinafter referred to as "emulsion-particles". When these emulsion particles have been grafted with a different, relatively rigid polymer, but still have a high rubber concentration, i.e. at least about 30 weight percent, these compositions are very suitable for blending with additional amounts of polymer, which may also contain rubber, to achieve a desired rubber content in the resultant composition. Such blendable intermediates are often referred to as "grafted rubber concentrates or "GRC's" and can be used to produce a wide variety of rubber-modified polymer compositions.
Under most circumstances, emulsion polymerization techniques are generally economically feasible only for the production of polymerized rubber particles having volume average diameters of less than about 0.3 micron. Larger particles can be made from these smaller polymerized rubber particles using other specific techniques, such as agglomeration.
Past research has been concentrated on obtaining ABS compositions having optimized physical properties by tailoring the rubber particle distributions (i.e., the sizes and types of rubber particles and the amounts of different size and/or type rubber particles) in the ABS and ABS-type compositions. See, for example, representative U.S. Pat. Nos. 3,509,237; 3,576,910; 3,652,721; 3,663,656; 3,825,621; 3,903,199; 3,903,200; 3,928,494; 3,928,495; 3,931,356; 4,009,226; 4,009,227; 4,017,559; 4,221,883; 4,224,419; 4,233,409; 4,250,271 and 4,277,574; wherein various "bimodal" particle size distributions are disclosed. As used in the art and herein, a composition having a "bimodal" particle size distribution contains two distinct groups of rubber particles, each group having a different average particle size. More recently, there have been disclosed "trimodal" rubber particle size rubber-reinforced polymer compositions wherein the rubber reinforcing ingredient takes the form of three distinct types of rubber particles.
U.S. Pat. No. 4,430,478 to Schmitt et al. and U.S. Pat. No. 4,713,420 to Henton disclose compositions, wherein relatively small and large emulsion rubber particles are used in combination with large mass rubber particles. The resulting "trimodal" compositions are characterized as having good combinations of toughness and gloss. However, Schmitt and Henton use mass rubber particles having a relatively large average particle diameter of at least 0.5 micron. Large mass particles are known to negatively affect the gloss and gloss sensitivity of the final product.
Additionally, U.S. Pat. No. 5,041,498 by Hare et al. discloses a trimodal composition which utilizes relatively small mass particles, i.e. 0.15 to 0.95 .mu.m. However, Hare's composition is limited to compositions containing less than 30 weight percent mass rubber based on the total weight of the rubber and a maximum of 14 percent rubber in the total composition. As is known in the art, emulsion polymers are more costly to produce than mass polymers, therefore large amounts of emulsion polymers render these compositions economically disadvantaged.
Therefore, there remains a need to develop a more economical ABS-type polymer composition having excellent balance of gloss, gloss sensitivity and impact properties, wherein a high percentage of small rubber particles are obtained from a mass process.